Transmitters generate signals. The heart of a transmitter is a signal generator. In the early days of radio, a radio receiver was made with a tunable filter. The difficulty of making a filter both tunable and extremely sharp, and extremely stable, has caused nearly all radios to use a super-heterodyne process because the hard-to-implement tunable filter is replaced with a simple highly optimized fixed tuned filter. The tuneability is gained via the use of a variable frequency signal generator. In a super-heterodyne receiver, the incoming signal is mixed with a variable frequency tone from a signal generator so as to shift its frequency to the narrow passed of a very sharp, fixed tuned, filter. As a result, the burden of tuneability and stability fall on the variable frequency signal generator.
One common waveform used in radar, sonar, and test instrumentation is a linear ramp in frequency, which is referred to as a chirp. Radars and instrumentation use linear chirp waveforms to obtain a resolving power that is, theoretically, proportional to the bandwidth of the chirp. For example, a radar with a 50 MHz bandwidth linear chirp can resolve two objects that are about 10 feet apart. A 500 MHz bandwidth linear chirp radar can resolve to objects that are about 1 foot apart. Because of the increased resolving power, there is great benefit to wider bandwidth capability. As the time bandwidth product grows, however, achieving the theoretical resolving power in practice becomes difficult due to the extremely high linearity required on the frequency ramp. It is an objective of this disclosure to show a method of implementing high time-bandwidth radar, sonar, and test instrumentation over extremely wide bandwidths, which are able to achieve near theoretically ideal performance, yet in a small footprint.
Another common waveform used in radar, sonar, and test instrumentation is stepped frequency waveforms. Generally, the system generates a tone at its transmit port for a short period of time which is long enough to excite some process being measured, like a device under test in the case of instrumentation, or in the case of radar or sonar, the various objects that reflect the energy during round trip time it takes the energy to travel from the radar or sonar to some distant point and back. Similar to the chirp, the resolving power is proportional, theoretically, to the bandwidth between the lowest frequency step and the highest frequency step.
Systems that use stepped frequency waveforms are hindered by the settling time that must occur at each frequency step. Until the new frequency has settled to a very high stability state, the system cannot transmit or receive a proper signal and therefore must cease functioning while a step is occurring. The net result is that there is a duty cycle reduction caused by the required settling time, and typically some performance degradation because in order to increase the duty cycle and save time, the system is allowed to run before a step is fully settled. For example, suppose that a system is stepping in frequency in 1 MHz steps, and is collecting data for 1 us at each frequency step. If one assumes a typical PLL oscillator takes 100 us to switch and settle to a new frequency, then the system spends about 99% of its time settling, and about 1% actually working. In other words, it has only a 1% duty cycle. In this example, the radar or sonar or instrumentation system could be made to operate nearly 100 times faster, or generate an SNR 100 times higher, if the settling time could be reduced to a small fraction of the 1 us duration needed at each frequency step. It is an objective of this disclosure to show a method of implementing radar, sonar, and test instrumentation capable of operating over extremely wide bandwidths with high duty cycle and achieving near ideal performance, yet in a small footprint.
Spread spectrum systems like secure communications and anti jam radars use frequency hopping as a means to spread their signal. While classical radar and test instrumentation typically make small incremental frequency steps that progressively go between the lowest and highest frequencies, the hopping used in spread spectrum systems requires the frequency steps to be taken randomly, including hopping quickly between the minimum and maximum frequency. Typically, the wider spans and random nature of the hopping causes the settling times to be worse than sequentially stepped systems, ultimately causing an even worse duty cycle. It is an objective of this disclosure to show a method of implementing frequency hopped spread-spectrum, radar, sonar, and test instrumentation capable of operating over extremely wide bandwidths with high duty cycle and achieving near ideal performance, yet in a small footprint.
Radar, sonar, communications, and test instrumentation sometimes use phase coding as a means of spectrum spreading and/or modulation and demodulation. It is an objective of this disclosure to show a method of implementing radar, sonar, communications and test instrumentation capable of using combinations of phase and frequency coding over extremely wide bandwidths, with high duty cycle and achieving near ideal performance, yet in a small footprint.
Signal generators take many forms, from (a) a basic analog oscillator circuit that oscillates at a frequency governed by an RC (resistor capacitor) network, or the resonance of an LC (inductor—capacitor) circuit or the resonance or delay of devices such as a SAW (surface acoustic wave) device, or a crystal, or a dielectric resonator, or BAW (bulk acoustic wave) device, etc., to (b) an analog oscillator followed by a non-linear stage that generates a harmonic term that is isolated by a filter to serve as the output tone, to (c) a combination of analog and digital circuits that may lock a higher frequency oscillator with poor long term stability but good short term stability, to a low frequency oscillator with good long term stability, but poor short term stability, such as phase-locked-loop (PLL), to (d) an NCO (numerically controlled oscillator) that is formed by driving a DAC (digital to analog converter) with data that causes the DAC to produce the desired output signal. Each method results in different sets of advantages and disadvantages in terms of metrics such as size, cost, weight, power consumption, settling time after a frequency change, modulation bandwidth, modulation linearity, long and short term stability, harmonic levels, spurious levels, and flexibility or ease of control.
Numerically controlled oscillators are well known to achieve optimally fast (short) settling times. At the same time, due to a combination of their discrete-time digitally-sampled stair-step output waveform, and the fact that the stair-step output levels must settle in each time-step period, an NCO cannot be clocked fast enough to produce the bandwidth required by many applications. It is an objective of this disclosure to show a method of obtaining arbitrarily large bandwidths.
Many applications are extremely sensitive to spurious signals. Applications such as radar, ladar (LAser Detection And Ranging), sonar, and numerous instrumentation applications, are fundamentally limited by the SFDR of the signals they generate and use to perform their function. The limit follows from the fact that they rely on correlations on these signals. Harmonic and spurious signals correlate along with the desired signal, causing an error in the measured correlation, which is supposed to only come from the desired signal.
One method of extending the bandwidth of an NCO is to use a frequency multiplier. This can be done using common mixer circuits or by using a circuit that generates harmonics and then filtering out all the harmonics that are undesired. This method results in a high noise floor and is unsuitable for many applications. Additionally, if the initial waveform was a complex waveform with, for example, a desired spectral notch, the multiplication process would destroy the notch. Another objective of this disclosure is to extend the bandwidth yet maintain a low noise floor, and preserve characteristics like spectral notches in the initial waveform.
Another method of extending the bandwidth of an NCO is to use a mixer together with a large N-way switch network and family of N tones spaced in frequency by the bandwidth (B) of the NCO. The switch network selects the desired tone and the NCO output signal is shifted in frequency by the frequency of the selected tone. In this way, the bandwidth covered by the output signal is N times the bandwidth of the NCO. This method results in a high power and a physically large and costly system that is incompatible with many applications. Another objective of this disclosure is to extend the bandwidth, yet at low power, a small footprint, and at relatively low cost.
Another method of extending the bandwidth of an NCO is a variant of the above paragraph, where the set of N oscillators is replaced by one or a smaller number of PLL synthesizers which are capable of being commanded to generate the N needed frequencies. The relatively slow switching speed of the PLL's in this solution, however, makes it unsuitable for many applications. It also prevents the system from continuously sweeping across the extended bandwidth since the PLL settling time is so long. Another objective of this disclosure is to extend the bandwidth at low power and small footprint, and with the capability to sweep the entire extended bandwidth continuously, and with optimally fast hopping or switching speed.